A polyurethane resin obtained by using a polyisocyanate consisting of an aliphatic diisocyanate or an alicyclic diisocyanate as a hardener is excellent in e.g., chemical resistance, flexibility and weather resistance. The polyisocyanate, when it is used as a polyurethane paint hardener, may be applied to a wide variety of substrates. When the polyisocyanate is applied to a flexible substrate such as a thin plate, a plastic film, cloth, leather and rubber, or when the polyisocyanate is applied to a hard substrate such as an aluminum plate, an iron plate, a copper plate and a stainless steel plate and thereafter purposely deformed, the coated film is required to have extensibility in order to follow deformation. For attaining such extensibility, a method of using a polyol as an auxiliary material to previously modify a polyisocyanate is suggested.
For example, Patent Literature 1 proposes an isocyanate composition using a polyester polyol as the polyol, whereas Patent Literature 2 proposes an isocyanate composition using a polyether polyol as the polyol.
In the meantime, a blocked polyisocyanate, which is a polyisocyanate having isocyanate groups blocked with a thermally dissociable blocking agent, is used. The blocked polyisocyanate, even if it is mixed with an active hydrogen compound (base resin) such as a polyol and a polyamine, does not react with the active hydrogen compound at normal temperature; however, if the blocked polyisocyanate is heated to dissociate a thermally dissociable blocking agent, thereby regenerating an isocyanate group, the reaction with an active hydrogen compound proceeds. Because of this, a base resin and a hardener can previously be stored as a mixture. As examples of the blocked polyisocyanate, Patent Literatures 3 and 4 propose blocked polyisocyanate compositions.